Descriptive Chemistry of Elements - OUSL block.pdfآ  1. Introduction to d-block elements Introduction

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  • 1 Published by The Open University of Sri Lanka

    2015

    Descriptive Chemistry of Elements

    d-Block

    Department of Chemistry The Open University of Sri Lanka

  • 2 Published by The Open University of Sri Lanka

    2015

    1. Introduction to d-block elements

    Introduction

    These lessons deal with the chemistry of d-block elements. The chemistry of d-block elements is

    very interesting and varies significantly from the chemistry of s- and p-block elements. This

    lesson introduces important concepts and terms such as electron configurations of metal centres,

    composition of coordination compounds, oxidation number, coordination number and geometries

    of coordination compounds that are needed to understand subsequent lessons.

    1.1 d-Block

    The filling of electrons into the d-levels creates the d-block. In a free metal, the nd-electrons fill

    after the filling of (n+1)s-electrons and before the filling of (n+1)p-electrons, thus, the d-block

    elements are situated between those of s- and p-block elements. Generally, the electron

    configuration of a d-block element can be represented as (n+1)s 2

    nd m

    or (n+1)s 1

    nd m

    where

    n = 3, 4, 5 or 6 and m = 1, 2, 3, …..or 10.

    When you look at the Periodic Table, you can see, that there are four series (or rows) in the

    d-block. They are the 3d, 4d, 5d and 6d-series and are in the 4 th

    , 5 th

    , 6 th

    and 7 th

    periods,

    respectively.

    The elements in the 3d-series (Z = 21 to 30)

    Element Sc Ti V Cr Mn Fe Co Ni Cu Zn

    Z 21 22 23 24 25 26 27 28 29 30

    The elements in the 4d-series (Z = 39 to 48)

    Element Y Zr Nb Mo Tc Ru Rh Pd Ag Cd

    Z 39 40 41 42 43 44 45 46 47 48

    The elements in the 5d-series (Z = 57, 72 to 80)

    Element La Hf Ta W Re Os Ir Pt Au Hg

    Z 57 72 73 74 75 76 77 787 79 80

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    The elements in the 6d-series (Z = 89, 104 to 112)

    Element Ac Rf Db Sg Bh Hs Mt Ds Rg Cn

    Z 89 104 105 106 107 108 109 110 111 112

    3d-elements and 4d-elements have atomic numbers 21-30 and 39-48, respectively. The elements

    listed in a vertical column belong to a Group, so that, Sc, Y, La and Ac belong to the same

    Group and it is called Group 3; similarly Cr, Mo and W belong to Group 6 and so on. At the

    Foundation Level we will concentrate more on the chemistry of 3d-elements.

    1.2 3d-Elements

    The filling of 3d-orbitals results in 3d-elements. We know that the electron configuration of an

    atom provides valuable information about the properties of an atom or its compounds. For

    example, the electron configuration of Ca = 1s 2 , 2s

    2 , 2p

    6 , 3s

    2 , 3p

    6 , 4s

    2 and calcium readily forms

    the Ca 2+

    ion.

    We know that the order of filling of energy levels is 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s,

    4f, 5d, 6p, 7s, etc. There are five 3d-orbitals and each orbital can have a maximum of two

    electrons. Let us look at the electron configurations of 3d-elements.

    1.3 Electron configuration of 3d-elements

    Scandium is the first 3d-element and its electron configuration is 1s 2 , 2s

    2 , 2p

    6 , 3s

    2 , 3p

    6 , 4s

    2 , 3d

    1 .

    The electron configurations and Group numbers of 3d-elements (Z = 21 to 30) are given below.

    Element (Name) Z E n

    configuration Group no.

    Sc (Scandium) 21 [Ar]4s 2

    3d 1 3

    Ti (Titanium) 22 [Ar]4s 2

    3d 2 4

    V (Vanadium) 23 [Ar]4s 2

    3d 3 5

    Cr (Chromium) 24 [Ar]4s 1

    3d 5 6

    Mn (Manganese) 25 [Ar]4s 2

    3d 5 7

    Fe (Iron) 26 [Ar]4s 2

    3d 6 8

    Co (Cobalt) 27 [Ar]4s 2

    3d 7 9

    Ni (Nickel) 28 [Ar]4s 2

    3d 8 10

    Cu (Copper) 29 [Ar]4s 1

    3d 10

    11

    Zn (Zinc) 30 [Ar]4s 2

    3d 10

    12

    Note that [Ar] = 1s 2

    2s 2

    2p 6

    3s 2

    3p 6 and also the electron configurations of Cr and Cu are

    [Ar]4s 1 3d

    5 and [Ar]4s

    1 3d

    10 , respectively.

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    Group number

    The Group number is equal to the sum of 4s and 3d electrons in the free metal (i.e. zerovalent

    metal or M 0 ). For example, the electron configuration of Sc is [Ar]4s

    2 3d

    1 or [Ar]3d

    1 4s

    2 , thus the

    Group number of Sc is 3. Therefore, the elements Sc, Y and La belong to Group 3 (formerly

    Group IIIB); the elements Fe, Ru, Os belong to Group 8 (formerly Group VIIIB) and the

    elements Zn, Cd, and Hg belong to Group 12 (formerly Group IIB).

    Activity

    1. Molybdenum (Mo) is a Group 6 metal in the 5 th

    period. Write the electron

    configuration of it.

    2. What is the electron configuration of the element with the atomic number 40?

    There are five 3d-orbitals and one 4s-orbital. The electron configurations of 3d-elements can also

    be represented as shown below where the distribution of valence electrons in the five 3d-orbitals

    and the 4s orbital are indicated; each square represents an orbital. The electron configurations of

    V, Cr, Mn, Fe, Ni and Cu are given below.

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    You can see that all these elements have unpaired electrons (e.g. V and Cr have 3 and 6

    unpaired electrons, respectively), thus, they are paramagnetic.

    Q : Why do we write electron configurations of Cr and Cu as [Ar]3d 5 4s

    1 and [Ar]3d

    10 4s

    1 ?

    A : There are five d-orbitals. The reason for this is that the exactly half-filled (i.e. d 5 )

    and completely filled (i.e d 10

    ) d-orbitals possess extra stability.

    Activity

    3. Give the electron distribution of the valence electrons in the five 3d-orbitals and the 4s

    orbital of zerovalent cobalt. Determine the number of unpaired electrons in it.

    1.4 Electron configuration of cations

    Removal of an electron(s) from a zerovalent metal (M 0 ) generates a cation. For example, V

    + ,

    V 2+

    , V 3+

    and V 4+

    can be formed, by removing 1, 2, 3 and 4 electron(s) respectively from V 0 .

    The electron configurations of V + , V

    2+, V

    3+ and V

    4+ ions are given below.

    Note that the 4s-electrons were removed before the removal of the 3d-electrons. The reason for

    this is that in a metal cation the energy of 3d-orbitals is lower than the energy of 4s-orbitals

    (c.f. when filling electrons, the energy of the 4s orbital is lower than 3d-orbitals). Thus the

    valence electron configuration of V + can be considered as 3d

    4 .

    V +

    = [Ar] 3d 3 4s

    1 = [Ar]

    V 2+

    = [Ar] 3d 3 4s

    0 = [Ar]

    V 3+

    = [Ar] 3d 2 4s

    0 = [Ar]

    V 4+

    = [Ar] 3d 1 4s

    0 = [Ar]

    4s 3d

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    Activity

    4. Determine the electron configurations of Co + and Co

    3+.

    1.5 Transition elements

    A transition element (M) should form at least one ion (M + , M

    2+ or M

    3+ ) with a partly filled

    d-shell. Transition metals show variable oxidation states and form coloured complex ions. Cu is

    a transition metal since it forms Cu 2+

    with the electron configuration [Ar]3d 9 4s

    0 , Cu also forms

    coloured compounds. Normally, Sc and Zn are not considered as transition elements as Sc forms

    only Sc 3+

    with no d-electrons (3d 0 ) whilst Zn forms only Zn

    2+ with a full d-sub shell (3d

    10 ). In

    some text books, Sc is considered as a transition metal because Zero valent scandium (Sc 0 ) has a

    partly filled d-shell, [Ar]3d 1 4s

    2 or [Ar]4s

    2 3d

    1 .

    1.6 Ligands

    A molecule or an ion with a donor atom(s) can act as a ligand. Some are neutral (e.g. NH3, CO,

    H2O) and some are negatively charged (Cl  , CN

     , CO3

    2 , SO4

    2 ).

    For example, NH3 can donate the lone-pair of electrons on N to a metal centre (Mn + ) to form a

    coordinate or dative bond.

    M n+

    + :NH3 M n+←:NH3 or [MNH3]

    n+ or [MNH3]

    n+

    Note that other ligands on M n+

    are not shown for clarity. In this case, the nitrogen (N) is the

    donor atom or the coordinating atom.

    1.7 Coordination Compounds

    Interaction of a metal centre or the central atom (M) with a number of ligands (L) gives a

    coordination compound as shown below.

    M + n L [MLn]

    The formula of a neutral coordination compound or complex ion is always written within square

    brackets [ ]. When we write the chemical formula of a coordination compound, first write the

    symbol of the metal, then the symbols of the negative ligands followe